1. Field of the Invention
The present invention relates to a laser light control system, and more particularly to a high-frequency current superposing circuit for supplying high-frequency current to a semiconductor laser, used as a light source in an optical pickup device for reading out information recorded on optical discs or the like.
Furthermore, the present invention relates to an optical pickup unit having the laser light control circuit and a light pickup device incorporating the laser light control circuit.
2. Description of the Related Art
Optical disc apparatus, such as CD-ROM drives, are widely used as external memory devices for personal computers. In particular, a DVD (Digital Versatile Disc) drive is attracting attention as the potential audio and video apparatus and data recording/reproducing apparatus of the next generation. The development and improvement of DVD devices is accompanied by demands for reducing the size and weight of such devices.
Among the elements from which the optical disc apparatus is comprised, an optical pickup device for reading information recorded on the disc, which comprises a holographic element, a semiconductor laser and a photodetector, has considerable volume and weight and consequently has a large ratio in relation to the overall apparatus. As a result, efforts to reduce the size and weight of the optical disc apparatus have mostly concentrated on the optical pickup device, achieving a variety of improvements to the conventional apparatus.
For example, Japanese Patent Laid-Open No. 3-269835 (1991) discloses a device in which the number of components in the unit has been reduced by positioning the photodetector on the optical path, enabling the size of the optical pickup device to be reduced.
The semiconductor laser, which is used as the light source for the optical pickup device, has the disadvantage of frequent return light noise. Return light noise occurs when a portion of light, which has been radiated from the semiconductor laser onto the surface of the optical disc and then reflected therefrom, returns to the semiconductor laser, thereby generating noise.
In order to solve this problem, a method has been proposed in which not less than 20 mA of high-frequency current, at a frequency of at least 100 MHz, is superposed onto the laser drive current. The circuit for supplying this high-frequency superposed current should preferably be located close to the semiconductor laser, so as to reduce the size of the optical pickup device. However, in general, high-frequency superposing circuits have a power voltage of 5V and a power consumption of not less than 100 mW. The heat which is consequently generated cannot be ignored. Furthermore, the light-generating properties of the semiconductor laser deteriorate at high temperatures, and especially become completely ineffective at temperatures exceeding 70.degree. C. Therefore, conventionally, it has not been possible to provide the high-frequency superposing circuit close to the semiconductor laser.